Thermotropic liquid crystalline polymers (LCPs) made from a partially aromatic polyester such as poly(ethylene terephthalate) (PET), and one or more aromatic hydroxy-carboxylic acids such a p-hydroxybenzoic acid (HBA) are well known, see for instance U.S. Pat. Nos. 3,778,410 and 3,804,805.
U.S. Pat. No. 4,892,912 describes the preparation of LCPs from a partially aromatic polyester, an acyloxyaromatic carboxylic acid, and a diacyloxyaromatic compound. The polymers produced are said to be more uniform in composition and can have higher molecular weights. The use of carboxylic acid anhydrides is not mentioned.
B. A. Yul'chibaev, et al., Vysokolekulyamye Soedineniya, Ser. B, vol. 37, (1995), pp. 166–171 describes the synthesis of LCPs from partially aromatic polyester, acetoxybenzoic acid, and diacetoxyhydroquinone in the presence of acetic acid. The use of carboxylic acid anhydrides is not mentioned.
U.S. Pat. No. 5,326,848 describes an LCP with repeat units derived from ethylene glycol (EG), 6-hydroxy-2-naphthoic acid (HNA), p-hydroxybenzoic acid (HBA), and terephthalic acid (T). The EG and T may be added “together” in the form of PET. It is mentioned that this LCP can be made by reaction of acyloxy derivatives of the EG, HNA and HBA, or that these compounds may be acylated in situ by using a carboxylic acid anhydride, but in the latter case a solvent is required to be present to produce high quality LCP. No mention is made of using a stoichiometric excess of any ingredient. This LCP is reported to have good oxygen barrier properties and to be useful in packaging.
Conventional (non-LCP) copolyester resins are known that provide low melt processing temperatures, isotropic properties, and good optical properties. This class of copolyester incorporates aliphatic moieties and alicyclics and consequently they exhibit high permeation properties (>160 cm3 oxygen 25 μm/m2 day atm).
Conventional LCP polyesters are known to provide good oxygen barrier properties, but they tend to be anisotropic and therefore weak in the direction transverse to melt flow, they have low elongation-to-break, and they tend to be opaque. The often also require high temperatures (>300 C) for melt processing. Some LCPs exhibit barrier values as low as 0.3 cm3 oxygen 25 μm/m2 day atm but require die head temperatures of 320° C. These properties tend to diminish their usefulness in many packaging applications as films and containers. Most packaging film processes require the barrier resin to be co-melt processed with structural resins and co-extrudable adhesives that can start to decompose at 275° C. The products of decomposition can introduce gel defects to extruded film or introduce odor or undesirable flavors to packaged food. It is therefore beneficial that the barrier resin be melt-processible into films and other articles below 275° C. and preferably below 230° C.